The present invention relates to rotary regenerative heat exchangers used as air preheaters and more specifically to the modification of the rotor construction to accommodate cold end heat exchange baskets with deep cold end heat exchange elements for performance upgrades and most specifically to tolerate ammonium bisulfate deposits in plants being retrofitted with ammonia-based systems for the selective reduction of nitrogen oxide emissions.
A rotary regenerative air preheater is employed to transfer heat from the hot flue gas stream to the cold combustion air in a steam generator plant. The rotor of the air preheater contains a mass of heat absorbent material which first rotates through a passageway for the hot flue gas steam where heat is absorbed by the heat absorbent material. As the rotor continues to turn, the heated absorbent material enters the passageway for the cold combustion air where the heat is transferred from the absorbent material to the cold air.
In a typical rotary regenerative air preheater, the cylindrical rotor is disposed on a vertical central rotor post and divided into a plurality of sectors by a plurality of radial partitions, referred to as diaphragms, extending from the rotor post to the outer peripheral shell of the rotor. Each of these sectors is divided by a plurality of stay plates extending between adjacent diaphragms into a plurality of compartments which are loaded with modular heat exchange baskets which contain the mass of heat absorbent material commonly formed of stacked plate-like heat exchange elements. It is common that the heat exchange elements can become fouled with deposits particularly in the cold end baskets where materials tend to condense and this deposit formation is exacerbated by the presence of ammonium bisulfate in the flue gas.
The reduction of nitrogen oxides (NO.sub.x) emissions from stationary combustion sources has become an important issue in most industrialized nations. As a result, the technology associated with the control of NO.sub.x emissions from fossil-fuel-fired steam generators has matured and expanded significantly. The NO.sub.x reduction processes available include both in-furnace NO.sub.x control and post-combustion NO.sub.x control primarily by selective catalytic or non-catalytic reduction. For example, with dry selective catalytic reduction systems, NO.sub.x reductions of 89 to 90 percent are achievable.
The selective catalytic reduction system uses a catalyst and ammonia gas as a reductant to dissociate NO.sub.x to nitrogen gas and water vapor. The catalytic process reactions are as follows: ##STR1##
Since NO.sub.x is approximately 95-percent NO in the flue-gas of steam generators, the first equation dominates.
The selective catalytic reduction reaction chamber is typically located between the economizer outlet and air preheater flue-gas inlet. This location is typical for steam-generating units with selective catalytic reduction operating temperatures of 575 to 750.degree. F. (300 to 400.degree. C.). Upstream of the selective catalytic reduction reaction chamber are ammonia injection pipes, nozzles, and a mixing grid. A diluted mixture of ammonia gas in air is dispersed into the flue-gas stream and distributed in the gas stream by the mixing grid. The ammonia/flue-gas mixture then enters the reaction chamber where the catalytic reaction is completed.
The selective non-catalytic reduction method predominantly utilizes ammonia gas or aqueous urea, CO(NH.sub.2).sub.2 as the reagent. The process is highly dependent upon flue-gas temperature and residence time for achieving high NO.sub.x removal efficiency. An effective temperature window ranging from 1600-2000.degree. F. (870-1090.degree. C.) is required for these systems. The primary chemical reaction for the two processes are as follows: EQU 4NO+4NH.sub.3 +O.sub.2.fwdarw.4N.sub.2 +6H.sub.2 O EQU CO(NH.sub.2).sub.2 +2NO+1/2O.sub.2.fwdarw.2N.sub.2 +CO.sub.2 +2H.sub.2 O
With either the catalytic or non-catalytic selective reduction processes, if SO.sub.3 is present in the flue gas either from the combustion process or from the catalytic oxidation of SO.sub.2 to SO.sub.3 and if there is unreacted ammonia from the selective reduction system, ammonium bisulfate forms by the following reaction: EQU NH.sub.3 +SO.sub.3 +H.sub.2 O.fwdarw.NH.sub.4 HSO.sub.4
This reaction takes place in a temperature range generally between 400 and 550.degree. F. and is dependent upon a number of parameters, including the concentrations of the constituents and temperature. The ammonium bisulfate will collect or condense on flyash particles and surfaces it comes in contact with including the heat transfer surfaces within the air preheater. Operating experience on high dust units indicates that the deposits collect within the air preheater on metal surfaces with temperatures generally between 300 and 375.degree. F. The deposit is in the form of a molten salt. In combination with flyash, the deposit is usually very sticky and difficult to remove. Experience has shown that for all regenerative air preheater designs and all ammonia slip levels, even as low as 1 ppm, there will be a measurable impact on pressure drop.
The consequences of ammonium bisulfate deposits on air preheater operation include the increase of pressure differential across the air preheater, an increase in air to gas leakage within the air preheater, a reduction in the thermal performance of the air preheater and an increase in the corrosion rates of heating elements, cold end structures and down stream equipment.
A typical design of an air preheater rotor contains multiple layers of heat transfer element baskets including one or more layers of hot end baskets at the top, one or more layers of intermediate temperature baskets and a layer of cold end baskets at the bottom. The stay plates previously mentioned extend from the upper hot end to the bottom of the intermediate baskets and have support bars welded to the bottoms which support the hot end and intermediate baskets which are loaded through the top of the rotor. The cold end baskets are supported by cold end rotor gratings and are loaded and unloaded radially through the side of the rotor. In a typical rotor, the stay plates with the basket support bars are approximately one inch above the cold end baskets. The cold end baskets typically have heat exchange element depth (height) of about 12 inches.
When a utility installs selective reduction equipment, ammonium bisulfate can condense and deposit on the heat transfer element surface through a given temperature range. This range can be over 75.degree. F. Given this temperature range, the ammonium bisulfate in a plant with a typical air preheater can be deposited not only in the cold end baskets but in multiple layers of heat transfer baskets including intermediate temperature baskets. This means that the replacement of cold end baskets may not be sufficient.
To reduce the problems associated with the deposition of ammonium bisulfate in multiple layers of baskets, the use of deep cold end baskets has proven useful. This allows the ammonium bisulfate to be captured primarily with a single layer of baskets and it moves the deposits closer to the cold end cleaning device, such as a sootblower, to thereby improve the effectiveness of the cleaning device in removing the deposits. If the ammonium bisulfate were allowed to collect in a region which bridges across two layers of baskets, the rate of accumulation and the subsequent consequences are dramatically accelerated. The use of deep cold end baskets has also been useful for general performance upgrades aside from the ammonium bisulfate deposit problem.
To allow the installation of a single cold end layer of baskets with a significantly increased element depth of as much as 42 inches or more compared to the typical depth of about 12 inches, the existing rotors must be modified. In the past, this has been done by first removing the cold end grating, removing the existing support bars on the bottom of the stay plates and the corresponding supports on the rotor shell and central post area. Stay plate extensions with bottom support bars are then added to the bottom of the existing stay plates to allow the cold end baskets to be deeper and move them closer to the cold end sootblower. New inner and outer supports are also added at the bottom. Although the cost of the added parts to make these modifications is not great, the labor for installation and the outage time for the plant can be very significant.